ИСТИНА

Gutless siboglinid tubeworms (frenulates, Osedax, Sclerolinum and vestimentiferans) relying their nutrition on bacterial endosymbionts is an annelid group of uncertain phylogenetic position. Orrhage and Müller compared the brains (a.k.a. supraesophageal ganglion) of a range of annelids and showed the general presence of 4 transverse commissures (though with different level of integration in the adult stage), connecting through their roots and circumesophageal connectives to the ventral nerve cords. Hitherto, brains of siboglinids remain understudied, except for the brain of the large-sized Vestimentifera, exceptionally shown to constitute a fusion of both the supra- and subesophageal ganglia. Studies of other siboglinids are required for tracing the ancestral state of brain and substructures. The brain anatomy of female Osedax priapus and Osedax ‘‘yellow collar’’ were reconstructed based on histology, 3D modeling, and immunohistochemistry combined with confocal microscopy. Osedax has a ventral intraepidermal brain, consisting of a large neuropile that ventrally and laterally is covered by densely packed perikarya. The neuropile contains a number of transverse commissures as well as longitudinal and diagonal neurite bundles. In the posteriomost brain are found a dorsal and a ventral commissural bundle with inter-decussating fibers, both of which connect with the ventral nerve cords. A dorsal and a ventral neurite bundle innervating each palp originate from two diagonal median commissures and two anterior commissures of the neuropile, respectively. The connection of these commissures to the ventral cords cannot be traced, due to the laterally disguising dense neuropile, also complicating the homology assessment of Osedax brain characters.